The present invention relates to spectral light sources, and more particularly to hollow-cathode type light sources.
Hollow-cathode spectral light sources are used in atomic absorption spectroscopy. Such light sources provide high intensity, sharply defined spectral lines for many analytical applications. A conventional hollow-cathode type light source design is seen in U.S. Pat. No. 3,264,511. The spectral light is produced as a result of a concentrated discharge which takes place between an anode and the hollowed portion of the cathode. The hollowed portion of the cathode contains the atomic species which generate the desired spectral light output. The discharge must be confined to the hollow portion of the cathode in order to achieve efficient operation. The teaching of U.S. Pat. No. 3,264,511 was that an insulating disc should be positioned between the anode and cathode, with the disc having a central aperture aligned with the hollow portion of the cathode. An insulating means is also taught as being disposed between the anode lead-in and the exterior of the cathode.
The insulating disc serves as the primary insulating means between the anode and the exterior of the cathode to insure that the discharge is confined to the cathode hollow. In order to facilitate assembly of the device, the insulating disc has a diameter which is less than the inside diameter of the glass envelope. There is thus some spacing between the outer edge of the disc and the glass envelope. This spacing is advantageous during evacuation of the envelope, since this is carried out through the base of the envelope.
For selected hollow-cathode devices, particularly for those where it is difficult to initiate the discharge, a relatively high starting voltage pulse, of for example 500-600 volts, is placed across the anode and cathode. The normal gas fill for such devices is several torr of inert gas such as argon. It has been found that with such high voltages some arcing takes place around the edges of the mica insulating disc between the anode and the exterior surface of the cathode, and this complicates starting the main discharge.